1. Field of the Invention
The present invention relates to hopped malt beverages, especially alcoholic brewery beverages produced at least in part from malt, and to means for improving the flavor stability thereof. More particularly, the present invention relates to imparting to hopped malt beverages improved stability against the production of thiols associated with the development of a xe2x80x9cskunkyxe2x80x9d odor and flavor in beer that has been exposed to ultra-violet or visible light.
2. Description of Related Art
The process of preparing fermented malt beverages, such as, beer, ale, porter, malt liquor, and other similar fermented alcoholic beverages, hereinafter referred to simply as xe2x80x9cbeerxe2x80x9d for convenience, is historically well established. As practiced in modern breweries, the process, in brief, comprises preparing a xe2x80x9cmashxe2x80x9d of malt, usually with cereal adjuncts, and heating the mash to solubilize the proteins and convert the starch into sugar and dextrins. The insoluble grains are filtered off and washed with hot water that is then combined with the soluble material. The resulting wort is boiled in a brew kettle to inactivate enzymes, sterilize the wort, extract desired hop components from added hops, and coagulate certain protein-like substances. The wort is then strained to remove spent hops and coagulum, cooled, pitched with yeast, and fermented. The fermented brew, known as xe2x80x9cgreenxe2x80x9d or xe2x80x9cruhxe2x80x9d beer, is then aged (xe2x80x9clageredxe2x80x9d) and clarified, filtered, and carbonated to produce the desired finished beer.
It is well known in the brewing art that when beer is exposed to light of wavelengths between about 300 nm and about 500 nm, the beer quickly develops a so-called xe2x80x9cskunkyxe2x80x9d flavor, which is sometimes also referred to as xe2x80x9csunstruckxe2x80x9d or xe2x80x9clight struckxe2x80x9d flavor. A considerable body of research has resulted in at least partial understanding of the chemical pathways involved. It is widely believed that the photochemical reaction sequence is initiated by the absorption of light by the presence of one or more of the several photosensitizers, e.g., flavins, especially, riboflavin, that are present in beer, which become excited, and, in their excited state, react with the bittering principles in the beer, isohumulones, to form 3-methyl butene thiol (3-MBT), which is understood to be the principal source of the skunky aroma. Only very small amounts of these sulfur-containing compounds are required to be present to impart the skunky flavor to the beverage and render it unacceptable. The riboflavin emanates mainly from the malt, to a minor extent via the hops used in the production of beer, and, according to common wisdom, the action of yeast during the fermentation. (See Tamer et al. Enzyme Microb Technology 10:754-56, December 1988.) This photochemical reaction is a problem that to some degree has been the subject of a diverse remediation.
More particularly, present understanding is that the reaction sequence is initiated by the absorption of light by riboflavin that is excited and, in its excited state, reacts with the bittering principles in beer, isohumulones, to form excited state isohumulones. The excited isohumulones then undergo a Norrish type I cleavage to give a 4-methyl-3-pentenoyl radical, which, in turn, fragments to carbon monoxide and a 3-methylbutenyl radical. The 3-methylbutenyl radical then reacts with cysteine or other suitable sulfur donors that may be present in the beer to form 3-MBT.
Excited isohumulones can, in principle, also be generated by direct absorption of ultraviolet light. The UV spectrum of trans isohumulone shows a small, but real, absorption of light of a wavelength between 300 nm and, approximately, 350 nm. Flint glass, although it absorbs all light below 300 nm, does not, therefore, completely protect isohumulones from direct UV excitation.
One approach that has been widely adopted relies on primary packaging coloration or opacity either to exclude light or, at least, exclude those wavelengths of light that are particularly problematic. Such attempts to prevent beverages from becoming skunky involve enclosing the beer in opaque cans or in bottles made of protective, i.e., colored, glassxe2x80x94brown or amber being the most efficient (see U.S. Pat. No. 2,452,968). These bottles reduce or eliminate the transmission to the beer of light having a wavelength shorter than about 560 nanometers. Such light is the most harmful because it assists the riboflavin in enhancing the production of the undesirable volatile sulfur compounds.
Brown bottle glass has become a standard for the brewing industry for avoiding the formation of skunky off-flavors, although in some circumstances green glass can be employed, generally with reduced efficacy.
Flint, or clear, glassxe2x80x94apart from the exclusion of a preponderance of ultraviolet wavelengthsxe2x80x94is ineffective as packaging for traditional beer products that are susceptible to the formation of skunky off-flavors on exposure to visible wavelengths, although its use is desirable in order to permit the consumer to view the contents of the package.
More recently, chemically reduced isohumulones have been used for bittering in lieu of natural isohumulones. (See Verzele, M., et al., U. Inst. Brew. 73:255-57, 1967.) Since skunky thiol is not formed from these compounds, the resulting beers do not develop light struck flavor when exposed to light. Unfortunately, the bittering properties of the reduced isohumulones are not identical to natural isohumulones and, hence, this approach results in flavor modification.
Other methods involve adding light-stabilizing materials to the beverage. (See U.S. Pat. No. 4,389,421.) However, in some jurisdictions, the use of such compounds has not been approved. Further, many brewers are reluctant to use any additives at all, but, rather, use hops or hop extracts in an effort to achieve traditional beer flavor.
Another alternative has been suggested in U.S. Pat. No. 4,389,421. This patent describes malt beverages that have added organic compounds possessing a 1,8-epoxy group and, optionally, another compound having a 1,4-epoxy group. The amount of the 1,8-epoxy compound is at least 0.25 ppb and, preferably, about one to six ppb by weight. Suitable sources of the 1,8-epoxy compounds are said to include 1,8-cineole, or plant essences from cardamom, eucalyptus, peppermint, lavender, laurel, or star anise. A suitable 1,4-epoxy compound is said to be 1,4-cineole. It is taught that the addition of these compounds prevents the development of the xe2x80x9clight struckxe2x80x9d flavor in a range of malt beverages (for example, beer, ale, malt liquors, etc.).
The problem of skunky flavor has been the subject of research for many years, and such research continues. (See Sakuma et al., xe2x80x9cSunstruck Flavor Formation in Beer,xe2x80x9d American Society of Brewing Chemists, Inc., 162-65, 1991). This article also deals with the part believed to be played by riboflavin in the reaction that produces the xe2x80x9cskunkyxe2x80x9d flavor and suggests that removing riboflavin from the finished beer may solve the problem.
U.S. Pat. Nos. 5,582,857 and 5,811,144 disclose a process for the production of a hopped malt beer wherein a processing liquid containing riboflavin is hopped to form the desired beverage. The improvement comprises subjecting that processing liquid to an effective amount of actinic radiation of a wavelength adapted to decompose the riboflavin and thereby reduce the amount thereof, whereby a beer having enhanced light stability is obtained. xe2x80x9cProcess liquidxe2x80x9d is defined as any unhopped wort or fermented wort (including green or bright beer) produced using malt.
U.S. Pat. No. 6,207,208, the disclosure of which is incorporated herein by reference in its entirety, is directed to the addition of an adsorbent clay to the process liquid of a brewing process to adsorb, and thereby remove, riboflavin present in the process liquid.
In that application, an improved process is provided for the production of a beer in which process a process liquid having a high riboflavin content is hopped to produce the desired beverage, wherein the improvement comprises treating the process liquid with an effective amount of a clay adsorbent, whereby the riboflavin content in the process liquid is reduced to less than 0.2 ppm, and the resulting beer has enhanced stability to light.
The clay employed as the adsorbent is a hydrated aluminum silicate or a hydrated aluminum-magnesium silicate. Examples of such clays that can be used include Fuller""s earth, bentonite, kaolinite, illite, and halloysite, as well as mixtures thereof. Preferably the clay is attapulgite, montmorillonite, or mixtures thereof.
The literature teaches a process in which sake, a unique Japanese fermented liquor or wine has been subjected to a light treatment. See Japanese published Patent Application 67667 entitled, xe2x80x9cProcess for Production of Sake Hardly Affected by Microbial Deterioration.xe2x80x9d This reference teaches treating finished sake with light in the 200 to 700 micron range, the object being to have the light decompose riboflavin that is present in the finished sake, which riboflavin is an essential nutrient for lactic bacteria that is responsible for infectious spoilage in finished sake. As a consequence of the riboflavin destruction, the growth of the bacteria is inhibited and the sake preserved. Sake is, of course, a totally different product from beer, being an unhopped nonmalt product, produced from rice using a starch-digestive enzyme amylase obtained from a mold and a special sake yeast to effect the fermentation. As stated, these differences materially distinguish sake from all malt brewery beverages. Note, in particular, that riboflavin in sake, absent any lactic acid bacterial infection, does not pose off-flavor development problems, such as those that are typically associated with hopped malt-containing products, such as beer.
As stated above, in accordance with currently accepted brewing science, the compound that is thought to be primarily responsible for the skunky off-flavor is 3-methyl-2-butene-1-thiol, although it has been suggested that hydrogen sulfide and methyl mercaptan, among other compounds, may also contribute. This compound is believed to be formed when photochemical cleavage of side chains of hop-derived isohumulones is followed by the reaction of the resulting 3-methyl-2-butenyl radical with an undetermined sulfur-containing compound, possibly methionine or, more likely, cysteine residues from the proteins that are normally present in the beer. Riboflavin, which is contributed from both vegetable, e.g., barley, and, to a much lesser extent, yeast sources, is generally accepted as being a photochemical sensitizer in this reaction sequence.
In accordance with the present invention, it has been found that if the reaction that produces the thiol compound responsible for the skunky flavor in beer production can be prevented or interrupted, then the resulting beverage will have enhanced stability against light.
One means by which a chemical reaction can be prevented or interrupted is by making one or more of its required reactants unavailable, for example, by its removal or destruction, by the blocking of its reactive site(s), or by its diversion to a competing reaction that produces innocuous products. Thus, the unavailability of one or more of the important reactants necessary for the thiol formation; e.g., the riboflavin, isohumulones, or the unidentified sulfur-containing compound; or a reactive intermediate derived therefrom, will prevent or, at least, reduce the production of skunky flavor in beer upon exposure to light. For example, U.S. Pat. No. 5,582,857, the disclosure of which is incorporated herein by reference in its entirety, discloses an improvement in a process for the production of a hopped malt beer in which a processing liquid containing riboflavin is hopped to form the desired beverage, wherein the improvement comprises subjecting the processing liquid to an effective amount of actinic radiation having a wavelength greater than 300 nanometers and adapted to decompose the riboflavin, thereby reducing the amount thereof and obtaining a beer having enhanced light stability.
U.S. Pat. No. 5,811,144 describes an improvement in a process for the production of a beer wherein wort having a high riboflavin content is hopped to produce the desired beverage, wherein the improvement comprises treating the wort with an effective amount of actinic radiation adapted to decompose riboflavin and having a wavelength of from 300 to 700 nanometers; wherein during the treatment with the actinic radiation, the wort is maintained at a temperature of from about 25xc2x0 C. to 90xc2x0 C. and is substantially anaerobic; whereby the riboflavin content is reduced to less than 0.2 ppm and the resulting beer has enhanced stability to light.
In a preferred embodiment, the present invention is directed to means for reducing riboflavin concentrations in beer in order to improve its light stability. According to the present invention, it has been found possible to stabilize beer to the effect of light by photolyzing various brewing intermediates. In particular, enhanced light stability has been obtained by riboflavin decomposition via photolysis of sweet wort under both aerobic and anaerobic conditions, photolysis of hopped wort under aerobic and anaerobic conditions, photolysis during fermentation, photolysis of unhopped beer under aerobic and anaerobic conditions, and photolysis of hopped beer under aerobic conditions. Additionally, the application of krausening to light struck beer and the fermentation of photolyzed wort with a mixture of yeast and lactic acid bacteria that require riboflavin for growth has been found beneficial.
Wort produced in the usual manner from malt(s) typically has a relatively high riboflavin content (for example, about 0.4 ppm or more). As used herein, riboflavin contents above 0.2 ppm are defined as xe2x80x9chigh.xe2x80x9d In accordance with one aspect of the present invention, the riboflavin content is reduced to less than 0.2 ppm, that level being defined for use herein as an xe2x80x9cinsignificantxe2x80x9d amount.
The present invention is directed to a process for the prevention of thiol formation in beer exposed to ultraviolet or visible light comprising removing or destroying at least one of the components present in the beer that are necessary for said thiol formation.
The formation of skunky thiol in beer follows clean first order kinetics. This means that a precursor (i.e., riboflavin) is being depleted via a first order pathway. When an unhopped beer is exposed to light under anaerobic conditions for an interval corresponding to that required for completion of skunky thiol formation, the riboflavin is depleted. This process will be referred to herein as xe2x80x9canaerobic photofinishing.xe2x80x9d Hops containing isoacids, i.e., isohumulones, can then be added and it has been found that no skunky thiol is generated upon subsequent light exposure. There are flavor changes in the beer associated with anaerobic photofinishing, but skunky thiol formation does not occur when the finished beer is exposed to visible light, i.e., wavelengths longer than about 390 nm.
Normally, isohumulone extracts are produced at a basic pH to improve the solubility of the extract. These, however, normally contain flavins and, while photochemical isomerization of basic isohumulone extracts is known in the art, it has now been found that an acidic extraction pH is necessary to avoid formation of lumichrome from the riboflavin so that subsequent photochemical isomerization results in a flavin-free extract suitable for use in this process.
The rate of photochemical riboflavin decomposition in beer and in wort is not significantly dependent on oxygen concentration. Oxygen, however, prevents skunky thiol development in beer during light exposure. It follows that riboflavin can be photochemically destroyed in a hopped beer without skunky thiol formation if an oxygen residual is maintained during the photolysis. This process will be referred to herein as xe2x80x9caerobic photofinishing.xe2x80x9d There are flavor changes in the beer associated with aerobic photofinishing, but skunky thiol formation does not occur when the finished beer is exposed to light under anaerobic conditions.
Further, skunky thiol is not produced when hopped wort is exposed to visible light during processing, and thus it follows that riboflavin can also be destroyed by photolysis during the fermentation step, either with or without oxygen injection.
In another aspect of the present invention, it has been found that the riboflavin can be destroyed by the action of bacteria. For example, beer fermentation employing a mixture of yeast and lactic acid bacteria results in a beer having a lower riboflavin content because the lactic acid bacteria that are present require riboflavin as a nutrient and, thus, take it up during the fermentation. This, again, results in a beer with enhanced light stability.
In yet another aspect of the present invention, the reactants in beer necessary to produce the skunky thiol, notably riboflavin, can be removed by adsorption to various surfaces. For example, in a method for the laboratory measurement of skunky thiol, it was found that the analysis can be carried out successfully only if the vessel used in the analysis is the bottle in which the beer went skunky or a vessel in which previous skunky thiol analyses had been performed. Consequently, when the vessel was cleaned by heating in a 400xc2x0 C. muffle furnace prior to use, the recovery of skunky thiol was reduced by 99+ percent. It is believed that this is caused by the binding of the thiol to the glass. Therefore, it follows that a beer rendered skunky can be made xe2x80x9cunskunkyxe2x80x9d by exposure to muffle furnaced glass, viz., by stripping skunky thiol from skunky beer by means of exposure of the beer or its headspace to muffle furnace activated ground glass. It is also expected that metal surfaces can similarly manifest such an adsorptive effect.
As employed herein, the term xe2x80x9cprocess liquidxe2x80x9d means any unhopped wort, fermented wort (including green or bright beer), or finished beer produced using malt.
Those skilled in the art will readily comprehend that the means for preventing the formation of skunky thiols from beer, or removing them, that have been mentioned above and others that may come to mind can in many instances be combined beneficially to yield even better results. For example, bottle glass, without anything more, has a typical green coloration. Flint is typically added to the glass in order to render the bottle glass optically clear. This practice has given rise to the expression xe2x80x9cflint glass.xe2x80x9d In the beer packaging industry, the term xe2x80x9cflint glass bottlesxe2x80x9d has become synonymous with xe2x80x9ccolorlessxe2x80x9d (i.e., free of visible color) glass bottles because flint is so much less expensive than other additives that are known to be fungible for this purpose. On the other hand, flint glass passes light at wavelengths that destabilize beer flavor chemistry. Cerium oxide glass and polycarbonate packaging are both known to block ultraviolet light. However, cerium oxide fluoresces, emitting light at riboflavin absorbing frequencies, which can also result in the development of the skunky off-flavor in hopped beer, thereby counting against its use as a primary packaging material for hopped beer. Polycarbonate, on the other hand, does not fluoresce as cerium glass does, but suffers from the drawback that it is normally applied as a coating to the bottle, creating problems both with respect to its application and with respect to recycling, especially in the case of returnable/reusable beer bottles. In accordance with the present invention, however, there is provided a beer packaged in a cerium oxide glass bottle, wherein the beer in the bottle has been riboflavin depleted by means such as those described herein. This means that the cerium oxide can replace the flint in the production of transparent glass and provide the added benefit of protecting the beer contained in these bottles from skunky flavor development via the ultraviolet route and, even though fluorescence occurs, the beer contained in the bottles does not develop skunky flavor because its riboflavin photochemistry has been disabled.
Thus, in one aspect the present invention provides a hopped malt beverage having enhanced light stability, with the beverage comprising hopped processing liquid, which processing liquid had a riboflavin content of less than 0.2 ppm during hopping.
In another aspect, the invention provides an improved process for the production of a beer in which a process liquid is hopped to produce the desired beverage, wherein the improvement comprises treating any process liquid having a high riboflavin content with an effective amount of actinic radiation having a wavelength greater than 300 nm and adapted to decompose riboflavin, wherein the riboflavin content is reduced to less than 0.2 ppm and the resulting beer has enhanced stability to light.
In another aspect, the present invention relates to an improved process for the production of a beer in which wort having a high riboflavin content is hopped to produce the desired beverage, wherein the improvement comprises treating the unhopped wort with an effective amount of actinic radiation adapted to decompose riboflavin and having a wavelength of from 300 to 700 nanometers; wherein, during said treatment with the actinic radiation, the wort is maintained at a temperature of from about 25xc2x0 C. to 90xc2x0 C. and is substantially anaerobic; whereby the riboflavin content is reduced to less than 0.2 ppm and the resulting beer has enhanced stability to light.
In another aspect, the present invention relates to an improved process for the production of a hopped malt beverage wherein a process liquid having a high riboflavin content is hopped to produce the desired beverage, wherein the improvement comprises treating the process liquid with an effective amount of actinic radiation from a lamp having a minimum power output specification as follows:
at least about 90 percent between about 300 and less than about 800 nanometers based on wavelength specific output power as measured in watts per square meter at a distance from the light source of three meters,
whereby the riboflavin content is reduced to less than about 0.2 ppm and the resulting hopped malt beverage has enhanced stability to light.
In still another aspect the present invention relates to a hopped malt beverage having enhanced light stability prepared by a process comprising treating a process liquid having a high riboflavin content with an effective amount of actinic radiation from a lamp having a minimum power output specification as follows:
at least about 90 percent between about 300 and less than about 800 nanometers based on wavelength specific output power as measured in watts per square meter at a distance from the light source of three meters,
whereby the riboflavin content is reduced to less than about 0.2 ppm and the resulting hopped malt beverage has enhanced stability to light.
In a preferred embodiment, the present invention is directed to an improved process for the production of a beer wherein a process liquid is hopped to produce the desired beverage, wherein the improvement comprises aerobically treating a process liquid for producing a beer having a high riboflavin content with an effective amount of actinic radiation having a wave length greater than 300 nanometers and adapted to decompose riboflavin, wherein the riboflavin content is reduced to less than 0.2 ppm and the resulting beer has enhanced stability to light.
In another preferred embodiment, the present invention is directed to a beer having enhanced stability to light produced by a process comprising treating a wort having a high riboflavin content with an effective amount of actinic radiation adapted to decompose riboflavin and having a wave length of from 300 to 700 nanometers; wherein, during the treatment with the actinic radiation, the wort is maintained at a temperature of from about 25xc2x0 C. to 90xc2x0 C. and is aerobic; whereby the riboflavin content is reduced to less than 0.2 ppm.